A plethora of sensors exist to indicate the presence or concentration of an analyte in a fluid or gas. Over the past years, different types of fiber optic moisture sensors have been developed. This detection of moisture is important in many industries and is especially useful in many applications in which free water is not present.
Generally, moisture sensors have a sensor head comprising a sensing material, an appropriate optical link, and a sensor readout. Sensing of the moisture is achieved by interaction of the water molecules with the sensing material. The optical link transmits light from a light source to the sensing material. The resulting optical signal is relayed to a light detector via the optical link so that a sensor readout can determine the concentration of the water in the environment being sensed.
Fiber optic sensors typically fall into one of the following four modes of sensors: transmissive sensors, porous fiber sensors, tip-coated (reflection) sensors, and side-coated (evanescent) sensors.
Transmission-mode sensors utilize two waveguides. The first launches light into the sensing medium or the fluid being tested and is collected by a second waveguide. The sensing medium is doped with a calorimetric or fluorescent compound that undergoes optical changes in the presence of the analyte being sensed. This type of mode is extremely inefficient and lossy due to the lack of optical couplings between the waveguides and the sensing medium.
A porous fiber sensor is made by altering the fiber core. In this mode of sensor, light propagates through the altered fiber core which has been doped with a water indicator. For example, U.S. Pat. No. 5,250,095 to Sigel, Jr., et al. began with a fiber of alkali borosilicate glass, a small section of which was heat treated to cause phase separation, resulting in a silica rich phase and an alkali borate rich phase. The latter was leached away, leaving only a porous silica core. The sensor is created by immersing the treated portion of the fiber in a solution containing an indicator. Due to the specially drawn fibers and chemical processing, this mode of sensor is costly and exhibits a slow response time.
In a reflective sensor, the fiber launches light onto a reflective or scattering sensing medium. Retroreflected light is collected by the same or an adjacent fiber. Reflection sensors are constructed either by coating the tip of the fiber with a cladding and indicator, then attaching a reflective film, or by coating a reflective surface with the indicator, then affixing the surface to the fiber end. The device may consist of either a bifurcated fiber, or a single fiber with a beam splitter, which separates the transmitted and detected rays. This mode of fiber sensor is fairly lossy.
The phenomenon of evanescence has frequently been employed to create sensors. In an evanescent sensor, light that is propagated down the waveguide is lost to the environment which is being sensed. The loss occurs over a length of the fiber that has a special cladding doped with an indicator. The cladding, made of materials such as sol-gel or electrostatic self-assembly (ESA) bilavers, has a higher index of refraction than the fiber core, causing the cladding to become the waveguide over the particular length of fiber. The light interacts with the indicator in the sensing medium, and as moisture is absorbed, the optical properties of the indicator change, changing the absorption of the spectrum. Evanescent-mode sensors require much processing and are typically very time-consuming to fabricate. Because the indicator is only present in the cladding, evanescent sensors lack desirable sensitivity.
Moisture sensors with sensor heads comprising superabsorbent polymers attached to a support substrate have also been employed. The superabsorbent polymer chemically binds a sensing reagent. Water from the sensed environment diffuses into the superabsorbent polymer until equilibrium is reached while the water molecules bond to the sensing reagent to change the optical quality of the polymer which is detected by a sensor readout.
Currently, superabsorbent polymers are placed on a support housing, or around glass support beads, or upon or around other means for support to form what generally can be called the sensor head in fiber optic moisture sensors. These means for support dictate against a direct optical connection of the polymer to the light source or light detector. As such, the general problems encountered with these sensors have been associated with the optical and physical coupling of optical links to the polymeric sensing elements, contributing to poor stability and reproducibility of results.
Therefore, it is one object of the invention to formulate a self-supporting optical quality superabsorbent polymer for use as the sensor head of a fiber-optic moisture sensor such that there can be direct physical and optical communication between the sensor head and an optical link allowing for accurate and reproducible results.
The described invention detects the presence of either low concentrations or high concentrations of water in an environment and consists of three primary components: a sensor head, an optical link, and a sensor readout.
The sensor head contains a sensing medium. The sensing medium in this invention is based on what is commonly referred to as a superabsorbing polymer, or hydrogel, doped with a hydrochromic material, otherwise generally know as a moisture sensing reagent. The superabsorbing polymer is hydrophilic (amenable to absorbing water) and serves as host for the hydrochromic material and propagates light along its length thereby acting as a waveguide.
The superabsorbing polymer composition of this invention creates an optical quality sensing media host which is self-supporting. Due to the versatility and strength of the superabsorbing polymer, it may be cast into a number of different satisfactory shapes (eg. fiber or cylinder) for use in a fiber optic moisture sensor, as such, the need for support means like a support housing or support beads is avoided. The self-supporting superabsorbing polymer may also be cast as a wave guide or light pathway on a photonic chip or the like.
The artisan will appreciate that one or more gradient index lenses may be adjoined to the superabsorbent polymer thereby directing a collimated beam of light through the polymer. Also, the artisan will appreciate that the sensor head further may consist of a reflective mirror which directs transmitted light back through the superabsorbent polymer to the transmitting optical link. In any of these configurations, the polymer is directly adjoined to whatever may be adjacent (eg. optical link, gradient index lenses, mirror) thereby maintaining quality optical transmissions.
Because the superabsorbing polymer is extremely hydrophilic, it will attract moisture from the environment it is sensing. This characteristic of attracting moisture increases the equilibrium concentration of water in the polymer thereby effectively magnifying the ability of the hydrochromic material to sense very small concentrations of moisture in the environment.
The preferred composition in accordance with the self-supporting superabsorbent polymer of this invention comprises a mixture of an acrylate resin, a polyacrylamide, a polymerization initiator, and at least one solvent. Preferably, the acrylate resin comprises 96% pure 2-hydroxyethyl acrylate, the polyacrylamide comprises 99% pure N,Nxe2x80x2-methylenebisacrylamide, the polymerization initiator comprises 2,2xe2x80x2-Azobis(2-(2-imidazolin-2-yl)propane) dihydrochloride, and the solvent comprises deionized water. Most preferably, 96% pure 2-hydroxyethyl acrylate comprises 27-43% by weight of the composition, 99% pure N,Nxe2x80x2-methylenebisacrylamide comprises 0.27%-0.42% by weight of the composition, deionized water comprises 56-72% by weight of the composition, and 2,2xe2x80x2-Azobis(2-(2-imidazolin-2-yl)propane) dihydrochloride comprises 0.02%-0.04% by weight of the composition.
As stated above, the superabsorbent material is doped with, or plays host to, a hydrochromic material. It is anticipated that there are many hydrochromic materials that can be used in this invention. This hydrochromic material may consist of various salts as well as polymer dyes. Preferably, the hydrochromic material comprises cobalt chloride or copper chloride. Cobalt chloride is most preferred. The amount of hydrochromic material used is 0.000005-0.06 grams per ml of deionized water. The actual concentration is strongly dependent on which hydrochromic material is used and for what range of moisture the sensor is designed to detect.
The self-supporting superabsorbent polymer constituents and the hydrochromic material is synthesized to form the doped self-supporting superabsorbent polymer useful in a fiber optic moisture sensor.
The next component, the optical link, is the means for transmitting light to and from the sensor head. This means for transmitting light includes at least one optical fiber by which light can be input to and output from the sensor head. As such, the optical link may comprise separate input and output optical fibers, one bifurcated optical fiber with branched input and output fibers, or some other variation thereof. Transparent optical fibers are most preferred. The optical link is adjoined to the sensor head such that there can be direct optical communication.
Lastly, the sensor readout is operatively associated with at least one light source and at least one light detector in optical communication with the optical link. In addition, the sensor readout has the necessary electronics to calibrate the optical signal from the sensor head to be able to determine the concentration of moisture in the environment. Finally, the sensor readout has some way of displaying or communicating the moisture concentration in the environment being sensed to the user or process control system.
By synthesizing the above described chemical compounds, we have been able to formulate a self-supporting superabsorbent polymer of excellent optical quality for use in a sensor head. This superabsorbent polymer may be cast as a fiber or cylinder, or as some other useful element, and does not require any support means as previously needed in fiber optic moisture sensors. It acts as the wave guide itself as opposed to prior art devices wherein the photons were carried along another light transport media such as a glass fiber or glass bead that served as a support substrate for the polymer. The superaborsent polymer may therefore be adjoined directly to an optical link thereby making possible a stable high-sensitivity, low-concentration moisture sensor with accurately reproducible results.
In a preferred embodiment of the invention, the sensor head of a fiber optic moisture sensor comprises a doped, optically transparent self-supporting superabsorbing polymer cast as a fiber. In another embodiment, the self-supporting superabsorbent polymer is opaque or optically non-transparent, as such, the polymer is described as a scattering superabsorbent polymer. The opaque nature of the self-supporting superabsorbent polymer allows for the use of a wider range of superabsorbent polymer materials in forming the self-supporting superabsorbent polymer.
In light of the above, it is an object of this invention to provide an optical sensor to sense water content in fluids.
It is an object of this invention to provide a self-supporting superabsorbing polymer integrated with a hydrochromic material as the sensing device.
It is an object of this invention to provide a reliable sensor with reproducible and repeatable results.
It is an object of this invention to provide a low-cost sensor that can easily be integrated into existing industrial systems.
It is an object of this invention to provide a method for making the sensor.